Add constexpr version of maximum to algorithm

Add the constexpr version of max which is only available in std since c++14.
Besides being a logical addition next to the already implemented constexpr version of min, there are actually other files, even in Swift/Basic itself, which re-implement this functionality, such as PrefixMap.h. Once this is implemented here, the functionality can be re-used in those other locations, instead of re-implemented each time.

include/swift/Basic/Algorithm.h

@@ -24,6 +24,12 @@ namespace swift {
   constexpr const T& min(const T &a, const T &b) {
     return !(b < a) ? a : b;
   }
+  
+  /// Returns the maximum of `a` and `b`, or `a` if they are equivalent.
+  template <typename T>
+  constexpr const T& max(const T &a, const T &b) {
+    return (a < b) ? b : a;
+  }
 } // end namespace swift
 
 #endif /* SWIFT_BASIC_ALGORITHM_H */